Accelerator

ABSTRACT

A pneumatic cylinder and piston actuator for successively accelerating and decelerating a structure to be tested. It includes a reservoir for gas under pressure, an accelerating cylinder and piston, and a decelerating cylinder and piston, the latter being of smaller diameter but longer stroke than the former. Upon release of a pneumatic pressure hold down, the accelerating piston, actuated by reservoir pressure, accelerates both pistons. Upon termination of the stroke of the accelerating piston the reservoir pressure decelerates the other piston, which is connected to the device to be tested. A non-return device terminates each such cycle, after which the actuator is reset.

United States Patent [1 1i Dye 1 Jan. 16, 1973 541 ACCELERATOR 2,819,589moss Geyer ..92/17 3,285,287 11/1966 Curran ...'92/62 X [75] l Kinnefllndanaimls' 3,309,062 3/1967 Jansz ..92/l7 [73] Assignee: General MotorsCorporation,

Detroit, Mich. Y Primary Examiner-Martin P. Schwadron i AssistantExaminerA. M. Ostrager v [22] filed 1970 Attorney -Paul Fitzpatrick andJean L Carpenter [21] Appl. No.: 89,823 [57] ABSTRACT [52] US. Cl...9l/41l A, 92/62, 92/85, A pneumatic cylinder and piston actuator forsucces- 92/ 152 sively accelerating and decelerating a structure to be[51] Int. Cl. ..Fl5b 11/06, FOlb 7/00, FOlb 11/02 I tested. It includesa reservoir for gas under pressure, [58] Field of Search ..92/61, 62,150, 151, 134, 85, an accelerating cylinder and piston, and adecelerating 92/17, 152; 91/411 A, 413, 170, 171 cylinder and piston,the latter being of smaller diame- I ter but longer stroke than theformer. Upon release of [56] References Cited a pneumatic pressure holddown, the accelerating piston, actuated by reservoir pressure,accelerates UNITED STATES PATENTS both pistons. Upon termination of thestroke of the ac- 3,018,762 1 /1962 Korb ..91/170 R Celerating Pistonthe reservoir Pressure decelerates the 3,559,538 2/1971 H ld 92/35 otherpiston, which is connected to the device to be 3,149,537 9/1964 Fink..92/62 X. tested. A non-retum device terminates each such cy- ,9 1933Greishabfir --9 cle, after which the actuator is reset. 3,173,659 3/1965Hemmeter .....92/17 X 2,782,766 2/1957 Bodem ..92/17 8 Claims, 5 DrawingFigures 22 20 62 71 k\\\\\x rg 50 7 w I;

I 4? 7g a 10- 76 \d\\% ia 59 44? I 39 ACCELERATOR The invention hereindescribed was made in the course of work. under a contract, orsubcontract thereunder, with the Department of Defense.

DESCRIPTION My invention relates to a fluid pressure device for firstaccelerating and then decelerating a load. It was conceived in responseto a requirement for an actuator to. impart a rapid pitching movement toan aircraft enapplications for smooth acceleration of considerable.

magnitude of a device to be tested.

The principal objects of myinvention are to provide apparatusparticularly suited to test structures and machines under strongtransient loads; a further object of the invention is to provide asimple, reliable, and conveniently operated accelerating anddecelerating actuator device; a further object is to provide a.substantially self-contained and reliable acceleratingdeceleratingmotor; a still further object of the device is to provide superior meansfor initiating the operation of a pneumatic actuator.

The nature of my invention and its advantages will be clear to thoseskilled in the art'from the succeeding detailed description andaccompanying drawings of the preferred embodiment of the invention.

However, by way of introduction to the description, it may be pointedout that, in its preferred embodiment, my invention involves anassemblyof a pressure vessel containing gas under pressure' and two coaxialcylinder and piston motors; an accelerating cylinder of relatively largediameter and short stroke and a decelerating cylinder of smallerdiameter and longer stroke. Upon release of the accelerating cylinder,its piston moves to push the piston of the decelerating cylinder throughthe stroke of the accelerating cylinder, after which, the reservoirpressure is effective to decelerate the decelerating piston. The pistonrod of-the decelerating cylinder thus is moved through a cycleconsisting of a substantially constant acceleration followed immediatelybysubstantially constant deceleration. The

device includes pneumatic pressure means for releasing or initiating thestroke of the accelerating piston and includes an arrangement forlocking the decelerating piston at the conclusion of its stroke and,further, pneucuits involved in control of the actuating device.

Referring first to FIG. 1, a support or stand 2 suptype, otherwise knownas Allison Model SOL The power plant includes a gas turbine engine and areduction gear rigidly coupled to the engine (not shown in thedrawings), and a propeller 4, which is driven. by the engine andsupported in known manner upon the reduction gear.

The nacelle is supported and the, thrust of the propeller is taken bytwo trunnions 6 rotatably mounted in bearings 7 at the top of the stand2. My invention relates to means for rotating the nacelle, en-- gine,and propeller about the axis of the trunnions 6 so I as to simulateloads placed on the power plant during abrupt pitching maneuvers, or thelike, of an aircraft in which the power plant is used.

The stand 2 is a static arrangement of tubing welded together bolted toa suitable base 8. The details of the stand are immaterial to myinvention,but it may be pointed out that at each side of the stand thereare vuprights l0 and 11 and braces 12, 14, 15, 16, and 18,

the purpose of these being to provide a suitably rigid and strongelevated support for the power plant.

The accelerating-decelerating .device which is the principal subject ofthis invention and which, for conciseness, will be called an actuator.is indicated at 20.

Externally it is a cylindrical pressure vessel comprising a first head22, a. second head 23, a diaphragm 24 between the heads, and cylindricalsections 26 and 27 defining a reservoir for gas under pressure. Thelower end of the actuator is connected through trunnions at 28 to'twoupstanding brackets 30 fixed to a member 3l of the stand 2. The parts ofthe actuator so far described are fixed together; they may be threadedor they may be held together by. through tie bolts. or held together inany manner generally considered suitable for pneumatic cylinders andpneumatic pressure vessels. The details are immaterial to my invention;

A piston rod 32 extending from the upper end of the actuator terminatesin a fitting 34 which is cross bored at 35 (see FIG. 2) to receive a pin36 extending from a structural member 38 fixed tothe nacelle 3 and theengine. By operating the actuator 20, the nacelle may be, rotated fromthe solid line position .as shown by the broken line renditions. I

Proceeding now to FIGS. 2 through 5 for anexplanation of the operationof the actuator 20, the cylindrical sections 26 and 27 and the heads 22and 23 define a reservoir 39 for compressed gas which is in two sectionspartially separated by the diaphragm 24. The two sections of thereservoir communicate freely through a ring of holes 40 in the diaphragm24. A'first cylinder 42 piston 44 and rod 32. A second piston rod 48fixed to piston 46 terminates in a disk or poppet 50 which acts ports inan elevated position a nacelle 3 within which is mounted a power plantof the well known military T56 as a trigger piston.

In the normal position of rest of the movable element comprising parts46, 47, 48. and 50, the poppet 50 engages a seat 51. mounted in the head22. As illustrated. the seat 51 is threaded so that by rotating the seatit is movable axially of the actuator for a purpose to plained.

The reservoir 39 is charged with gas under pressure; specifically, inthe application referred to, nitrogen at 2400 psi. Since the cylinder 42is open, this pressure is exerted against the left face of piston 46.The reservoir 39 may be charged from an external high pressure reservoir52 which is pressurized by a pump 54. The nitrogen (or other gas, ifpreferred) is supplied from reservoir52 through a line 55 and a shutofivalve 56 to port'5 7 to charge the reservoir 39 to the desired pressurelevel, after which the valve 56 is closed. Valve 56 may be of a pressureregulating type, or reservoir pressure may be controlled manually. Thispressure is applied to the differential piston comprising piston 46 andbe expoppet 50. Since the port 57'aIs0 communicates with the space 58 inthe interior of cylinder 43, the reservoir pressure is exerted againstthe right hand face of piston 44 as illustrated. The space 60 withincylinder'42 to the right of piston 46 as illustrated is a buffer orcushion space. Space 60 is charged to a proper pressure, about 7 psig inthe example being described,,through' a charging port 62. As shown inFIG- 5, charging port 62 is The capacity of reservoir 39 is preferablysuch that no charging port 67 with a three-way valve 68 by which chamber66 may be fed gas under pressure or may be connected to exhaust line 64.

High pressure gas line 55 is also connected through a regulating valve70 and a three-way valve 71 to a port 72 extending through the seat 51and terminating in a pressure chamber 74 closed, in the position of theparts systems with suitable gas tight joints, piston rings, seals forthe piston rod, and such, andpressure gauges,

' remote valve controls, and the like, as'desired, but

details of this sort are a mere matter of conventional design and notmaterial to an understanding of my invention.

Proceeding now to the mode of operation of the'actuator 20, the cycleisbegun with the parts in the positions shown in FIG. 2. The reservoir ischarged in the particular example to 2400 psi, cushion space 60 ischarged to 7 psig, and the space 66 and pressure chamber 74 are vented.Pressure inreservoir 39 acting on piston 44 and poppet biases them tothe left as illustrated, and the same pressure acting on the left faceof piston 46 biases it to the right as illustrated. In the preferredembodiment, the effective area of piston 44, that is the area of thepiston less thearea of piston rod 1 32, is half thatof piston 46, whichis piston 46 area less rod 47 area. The .left facefof piston 44 isvented. The

travel and the trigger piston 50 is seated over chamber 74, closing thechamber. To initiate a stroke of the apdicated in FIG. 3, the highreservoir pressure is applied to the left face of this piston andthusthe force.

becomes highly unbalanced, since the piston 50 no longer has anyefi'ective pressure difference exerted on it. Thus, the large piston 46forces the small piston 44 with it, projecting the rod 32 with highacceleration.

great reservoir pressure drop, preferably not over five from cylinder 43intothe reservoir 39.

As piston 46 moves to the right, it compresses the air within thecushion chamber 60, the pressure rising slowly with displacement atfirst and then rapidly and, at some point, the piston 46 is deceleratedand is stopped short before bumping into diaphragm 24'. Piston 46 thenmay oscillate and come to rest near diaphragm 24. r

Since piston 44 is at or near atmospheric pressure at its left face andis working against reservoir pressure at its right face, the piston isthen=rapidly decelerated. Since, as stated, the effective area of piston44 is half. that of piston 46, it equals the differential of areabetween pistons 46, 44. ,As a result, the piston 44 is decelerated byreservoir pressure at a rate substantially equal to the prioracceleration until it reaches theend of its stroke, which must occurbefore the piston 44 col- 1 lides with head 23.

The piston 44 is carried ahead by the inertia of the nacelle 3 and itscontents. FIG. 4 illustrates the'termination of the acceleration strokeand the beginning of the deceleration portion of the stroke. It will benoted that piston 44 has parted from piston rod 47.

Some means is needed. ofcourse, to preventthe reservoir pressure fromdriving piston 44 in the reverse direction to strike forcibly againstpiston $441; This is quite simply and satisfactorily provided by theone-way stop or ratchet arrangement 82 shown in FIG. l. A rectangularstrut83 pivoted to the structural member 38 on the nacelle slidesthrough a guide 86 mounted on the stand 2. This strut is provided withratchetteeth 87 on two faces, which teeth are engageable by springurgedpawls 88.The pawls allow-the strut 83 to move it and extend the pistonrod 32 slightlyso as to free the pawls 88'. The pawls then are retractedbysuitable mechanism, which is immaterial to my invention, and heldclear of the ratchet teeth 87." Valve 67 is then actuated to bleedpressure outof the chairiber 66, so that the reservoir pressure. exertedagainst the right face of piston 44 pushes it against the end of rod 47.The space to the left of piston 44 is then fully vented. Under theseconditions, the reservoir pressure acting upon piston 46 retains theparts in intermediate position.

The next step is to actuate valve 63 to force gas into chamber 60 andthus drive piston 46 to the left until trigger piston 50 is seated.Valve 71 is then actuated to reduce pressure in chamber 74 so that thetrigger piston 50 is inactive. Valve 63 is then actuated to reduce thepressure in cushion chamber 60 to the appropriate value to snub thepiston 46 on its next stroke. The ratchet pawls 88 are then put inposition to be effective.

The pressure regulating valve 70 is set slightly above thetriggeringpressure in chamber 74. It prevents undesired flow when thetrigger piston is unseated.

Because of the elastic properties of the gas and the sufficient capacityto the reservoir 39, the actuator is given a substantially linearacceleration followed by deceleration of substantially the samemagnitude. The parts under test may be instrumented with strain gaugesor otherwise to measure the effect of this acceleration and decelerationuponthem. Also, the parts may be tested for failure if there is anystructural deficiency. It

I will be noted that the rate of acceleration is a function firstcylinder with a piston rod extending into the second cylinder; a pistonslidable in the second cylinder with a piston rod extending from thesecond cylinder in the direction away from the first cylinder; thesecond piston having approximately half the effective area of the first;means to apply a fluid pressure in opposition to both pistons toaccelerate both pistons through the stroke of the first piston, means tostop the first piston, and means to apply the fluid pressure to thesecond piston to decelerate the second piston during overtravel beyondthe stroke of the first piston, the means toapply the fluid pressurecomprising a fluid pressure reservoir forming an integral assembly withthe said cylinders.

almost entirely of two variables, the inertia or moment of inertia ofthe load and the pressure to which the reservoir 39 is charged. Thus,the acceleration may be greatly varied and may be smoothly varied over'awide range. Also, by varying the position of seat 51, the starting pointof the stroke may be varied so that the length of the stroke is variableby infinitesimal increments.

It should be clear from the foregoing to those skilled in the art that lhave devised an extremely simple and effective force-exerting device forthe purpose described and one which may be scaled to suit any particularinstallation and which may be employed to actuate linearly, or rotate,any type of device under test.

The particular example described herein is quite large, being capable ofexerting 100,000 pounds force when charged to 2500 psi and having astroke of four feet.

The detailed description of the preferred embodiment of the inventionfor the purpose of explaining the principles thereof is not to beconsidered as limiting or restricting the invention, as manymodifications may be made by the exercise of skill in the art withoutdeparting from the spirit of the invention.

lclaim:

1. A motor system for imparting a controlled acceleration-decelerationcycle to a device to be tested comprising, in combination, a firstcylinder; a second cylinder coaxial with the first; a piston slidable inthe first cylinder with a piston rod extending into the second cylinder;a piston slidable in the second cylinder with a piston rod extendingfrom the second cylinder in the direction away from the first cylinder;the second piston having approximately half the effective area of thefirst; means to apply a fluid pressure in opposition to both pistons toaccelerate both pistons through the stroke of the first piston, means tostop the first piston, and means to apply the fluid pressure to thesecond piston to decelerate. the second piston during overtravel beyondthe stroke of the first piston.

3. A motor system for imparting a controlled acceleration-decelerationcycle to a device to be tested comprising, in combination, first andsecond expansible-chamber motors, each motor including a cylinder andmeans movable with respect to the cylinder including a piston and apiston rod; the cylinders being fixed together and the movable means ofthe first motor being engageable with the movable means of the secondmotor; means for supplying fluid under pressure to the first motor toaccelerate both movable means; a trigger piston fixed to the movablemeans of the first cylinder and normally exposed to the pressure of thefluid to hold the movable means'in an initial position;

means to apply fluid pressure to the other face'of the trigger piston toinitiate movement of the movable means; means for stopping thefirstmovable means; and means to apply fluid pressure to the second movablemeans to decelerate the second movable means during its overrun from thefirst movable means; 'the cylinders and the second movable means beingconnectable between elements to be relatively accelerated.

4. A pneumatic motor system for successively accelerating anddecelerating a load comprising, in combination, a pressure vesselincluding first and second heads and a diaphragm intermediate the heads,the vessel defining a reservoir for gas under pressure; a first cylinderextending from the diaphragm'and having an opening into the reservoir;21 second cylinder coaxial with the first and extending from thediaphragm to the second head, the second cylinder being of less area andlonger than the first; a first movable element including a pistonslidable in the first cylinder, arod extending from the first'cylinderinto the second, and a poppet engageable with the first head; a secondmovable element engageable with the first movable element and includinga piston slidable in the second cylinder and a rod extending from thesecond cylinder; means for applying pressure through the first headagainst the poppet to initiate a stroke of the first piston and therebyaccelerate the second piston; and means providing communication betweenthe reservoir and the second cylinder to oppose the force of the firstpiston and to decelerate the second piston after termination of thestroke of the first piston.

5. A pneumatic motor system for successively accelerating anddecelerating a load comprising, in combination, a pressure vesselincluding first and second heads and a diaphragm intermediate the heads,the vessel defining a reservoir for gas under pressure; a first cylinderextending from the diaphragm and having an opening into the reservoir; asecond cylinder coaxial with the first and extending from the diaphragmto the second head, the second cylinder being of less area and longerthan the first; a first movable element including a piston slidable inthe first cylinder and a rod extending from the first cylinder into thesecond; a second movable element engageable with the first movableelement and including a piston slidable in the second cylinder and a rodextending from the second cylinder; means to initiate a stroke of thefirst piston and thereby accelerate the second piston; means providingcommunication between the reservoir and the second cylinder to opposethe force of the first piston and to decelerate the second piston aftertermination of the stroke of the first piston; means effective to holdthe second piston upon termination of its stroke; and means forcontrollably returning both pistons to their initial positions.

6. A pneumatic motor system for successively accelerating anddecelerating a load comprising, in combination, a pressure vesselincluding first and second heads and a diaphragm intermediate the heads,the vessel defining a reservoir for gas under pressure; a first cylinderextending from the diaphragm and having an opening into the reservoir; asecond cylinder coaxial with the first and extending from the diaphragmto the second head, the second cylinder being of less area and longerthan the first; a first movable element including a piston slidable inthe first cylinder, a rod extending from the first cylinder into thesecond, and a poppet engageable with the first head; a second movableelement engageable with the first movable element and includ ing apiston slidable in the second cylinder and a rod extending from thesecond cylinder; means for applying pressure through the first headagainst the poppet to initiate a stroke of the first piston and therebyaccelerate second piston after termination of the stroke of the firstpiston; means effective to hold the second piston upon termination ofits stroke; and means for controllably returning both pistons to theirinitial positions.

7. A motor system for imparting a controlled acceleration-decelerationcycle to a device to be tested comprising, in combination, a firstcylinder; a second cylinder coaxial with the first and of smallerdiameter than the first; apiston slidable in the first cylinder with apiston rod extending into the second cylinder; a piston' slidable in thesecond cylinder with a piston rod extending from the second cylinder toprovide an output from the system; the second piston having a longerstroke than the first; means to apply a fluid under pressure inopposition to both pistons to accelerate both pistons through the strokeof the first piston, and means to apply the fluid under pressure only tothe second piston to decelerate the second piston during overtravelbeyond the stroke of the first piston.

8, A motor systemfor imparting acontrolledacceleration-decelerationcycle to a device to be testedcomprising, in combination, a first cylinder; a second cylinder coaxialwith the first and of smaller diameter than the first a piston slidablein the first cylinder with a piston rod extending into the secondcylinder; a piston slidable in the second'cylinder with a piston rodextending from the second cylinder in the direction away from the firstcylinder to provide an output from the system; the second piston havinga longer stroke than the first; means to apply a fluid under pressure inopposition to both pistons to accelerate both pistons through the strokeof the first piston, and means to apply the fluid under pressure only tothe second piston to decelerate the second piston during overtravelbeyond the stroke of the first piston.

1. A motor system for imparting a controlled accelerationdecelerationcycle to a device to be tested comprising, in combination, a firstcylinder; a second cylinder coaxial with the first; a piston slidable inthe first cylinder with a piston rod extending into the second cylinder;a piston slidable in the second cylinder with a piston rod extendingfrom the second cylinder in the direction away from the first cylinder;the second piston having approximately half the effective area of thefirst; means to apply a fluid pressure in opposition to both pistons toaccelerate both pistons through the stroke of the first piston, means tostop the first piston, and means to apply the fluid pressure to thesecond piston to decelerate the second piston during overtravel beyondthe stroke of the first piston.
 2. A motor system for imparting acontrolled acceleration-deceleration cycle to a device to be testedcomprising, in combination, a first cylinder; a second cylinder coaxialwith the first; a piston slidable in the first cylinder with a pistonrod extending into the second cylinder; a piston slidable in the secondcylinder with a piston rod extending from the second cylinder in thedirection away from the first cylinder; the second piston havingapproximately half the effective area of the first; means to apply afluid pressure in opposition to both pistons to accelerate both pistonsthrough the stroke of the first piston, means to stop the first piston,and means to apply the fluid pressure to the second piston to deceleratethe second piston during overtravel beyond the stroke of the firstpiston, the means to apply the fluid pressure comprising a fluidpressure reservoir forming an integral assembly with the said cylinders.3. A motor system for imparting a controlled acceleration-decelerationcycle to a device to be tested comprising, in combination, first andsecond expansible-chamber motors, each motor including a cylinder andmeans movable with respect to the cylinder including a piston and apiston rod; the cylinders being fixed together and the movable means ofthe first motor being engageable with the movable means of the secondmotor; means for supplying fluid under pressure to the first motor toaccelerate both movable means; a trigger piston fixed to the movablemeans of the first cylinder and normally exposed to the pressure of thefluid to hold the movable means in an initial position; means to applyflUid pressure to the other face of the trigger piston to initiatemovement of the movable means; means for stopping the first movablemeans; and means to apply fluid pressure to the second movable means todecelerate the second movable means during its overrun from the firstmovable means; the cylinders and the second movable means beingconnectable between elements to be relatively accelerated.
 4. Apneumatic motor system for successively accelerating and decelerating aload comprising, in combination, a pressure vessel including first andsecond heads and a diaphragm intermediate the heads, the vessel defininga reservoir for gas under pressure; a first cylinder extending from thediaphragm and having an opening into the reservoir; a second cylindercoaxial with the first and extending from the diaphragm to the secondhead, the second cylinder being of less area and longer than the first;a first movable element including a piston slidable in the firstcylinder, a rod extending from the first cylinder into the second, and apoppet engageable with the first head; a second movable elementengageable with the first movable element and including a pistonslidable in the second cylinder and a rod extending from the secondcylinder; means for applying pressure through the first head against thepoppet to initiate a stroke of the first piston and thereby acceleratethe second piston; and means providing communication between thereservoir and the second cylinder to oppose the force of the firstpiston and to decelerate the second piston after termination of thestroke of the first piston.
 5. A pneumatic motor system for successivelyaccelerating and decelerating a load comprising, in combination, apressure vessel including first and second heads and a diaphragmintermediate the heads, the vessel defining a reservoir for gas underpressure; a first cylinder extending from the diaphragm and having anopening into the reservoir; a second cylinder coaxial with the first andextending from the diaphragm to the second head, the second cylinderbeing of less area and longer than the first; a first movable elementincluding a piston slidable in the first cylinder and a rod extendingfrom the first cylinder into the second; a second movable elementengageable with the first movable element and including a pistonslidable in the second cylinder and a rod extending from the secondcylinder; means to initiate a stroke of the first piston and therebyaccelerate the second piston; means providing communication between thereservoir and the second cylinder to oppose the force of the firstpiston and to decelerate the second piston after termination of thestroke of the first piston; means effective to hold the second pistonupon termination of its stroke; and means for controllably returningboth pistons to their initial positions.
 6. A pneumatic motor system forsuccessively accelerating and decelerating a load comprising, incombination, a pressure vessel including first and second heads and adiaphragm intermediate the heads, the vessel defining a reservoir forgas under pressure; a first cylinder extending from the diaphragm andhaving an opening into the reservoir; a second cylinder coaxial with thefirst and extending from the diaphragm to the second head, the secondcylinder being of less area and longer than the first; a first movableelement including a piston slidable in the first cylinder, a rodextending from the first cylinder into the second, and a poppetengageable with the first head; a second movable element engageable withthe first movable element and including a piston slidable in the secondcylinder and a rod extending from the second cylinder; means forapplying pressure through the first head against the poppet to initiatea stroke of the first piston and thereby accelerate the second piston;means providing communication between the reservoir and the secondcylinder to oppose the force of the first piston and to decelerate thesecond piston after termination of The stroke of the first piston; meanseffective to hold the second piston upon termination of its stroke; andmeans for controllably returning both pistons to their initialpositions.
 7. A motor system for imparting a controlledacceleration-deceleration cycle to a device to be tested comprising, incombination, a first cylinder; a second cylinder coaxial with the firstand of smaller diameter than the first; a piston slidable in the firstcylinder with a piston rod extending into the second cylinder; a pistonslidable in the second cylinder with a piston rod extending from thesecond cylinder to provide an output from the system; the second pistonhaving a longer stroke than the first; means to apply a fluid underpressure in opposition to both pistons to accelerate both pistonsthrough the stroke of the first piston, and means to apply the fluidunder pressure only to the second piston to decelerate the second pistonduring overtravel beyond the stroke of the first piston.
 8. A motorsystem for imparting a controlled acceleration-deceleration cycle to adevice to be tested comprising, in combination, a first cylinder; asecond cylinder coaxial with the first and of smaller diameter than thefirst; a piston slidable in the first cylinder with a piston rodextending into the second cylinder; a piston slidable in the secondcylinder with a piston rod extending from the second cylinder in thedirection away from the first cylinder to provide an output from thesystem; the second piston having a longer stroke than the first; meansto apply a fluid under pressure in opposition to both pistons toaccelerate both pistons through the stroke of the first piston, andmeans to apply the fluid under pressure only to the second piston todecelerate the second piston during overtravel beyond the stroke of thefirst piston.